AMD R9 270X gives "Display driver stopped responding and has recovered" error in Windows 7

My new X51 R2 gets the "Display driver stopped responding and has been recovered" Windows 7 errors when screensaver (Mystify or Ribbons) is running. It happens randomly - sooner or later when screen saver is running. Then the screen saver process hangs and can be closed via task manager.

When the screen goes off (if the system is idle for a period of time), the process dwm.exe eats ~ 50% CPU time and this is accompanied by a loud noise from the CPU fan. A Windows re-install (all the drivers arev from the Dell website) does not resolve the problem. So far the mainboard has been replaced one time, the R9 270X video card - two times. A fresh Windows 7 installation with all updates did not help.

I already have tried the lastest AMD Catalyst driver whql, the TDR fix from Microsoft - to no avail. At this point I am at a loss. Spoke with Dell support over the phone for several hours - they can't help me either. Does any one else experience a similar problem?